ILIUM

 

The Ilium or the flank forms the upper extended plate-like part of the hip bone. It has upper and lower parts and three surfaces. The upper end is called the iliac crest which forms the two-fifths of the acetabulum and the lower end which is smaller than upper end is fused with pubis and ischium at the acetabulum. The upper part is much expanded, and has gluteal, sacropelvic and iliac (internal) surfaces. The posterolateral gluteal surface is an extensive rough area; the anteromedial iliac fossa is smooth and concave; and the sacropelvic surface is medial and posteroinferior to the fossa, from which it is separated by the medial border.

Ilium

Ilium

 

Iliac crest

The iliac crest is the superior border of the ilium. It is broad and convex upwards but sinuous from side to side, being internally concave in front and convex behind. Its ends project as anterior and posterior superior iliac spines. The anterior superior iliac spine is palpable at the lateral end of the inguinal fold; the lateral end of the inguinal ligament is attached to the anterior superior iliac spine. The posterior superior iliac spine is not palpable but is often indicated by a dimple, approximately 4 cm lateral to the second sacral spinous process, above the medial gluteal region.

The iliac crest has ventral and dorsal segments. The ventral segment occupies slightly more than the anterior two-thirds of the iliac crest. It has internal and external lips and a rough intermediate zone that is narrowest centrally. The dorsal segment, which occupies approximately the posterior 1/3^rd in humans. It has two sloping surfaces separated by a longitudinal ridge ending at the posterior superior spine. The tubercle of the iliac crest projects outwards from the outer lip approximately 5 cm posterosuperior to the anterior superior spine. The summit of the iliac crest, a little behind its midpoint, is level with the 4^th lumbar vertebral body in adults and with the 5^th lumbar vertebral body in children aged 10 years or less.

Anterior border

The anterior border descends to the acetabulum from the anterior superior spine. Superiorly it is concave forwards. Inferiorly, immediately above the acetabulum, is a rough anterior inferior iliac spine, which is divided indistinctly into an upper area for the straight head of rectus femoris and a lower area extending laterally along the upper acetabular margin to form a triangular impression for the proximal end of the iliofemoral ligament.

Posterior border

The posterior border is irregularly curved and descends from the posterior superior spine, at first forwards, with a posterior concavity forming a small notch. At the lower end of the notch is a wide, low projection known as the posterior inferior iliac spine. Here the border turns almost horizontally forwards for approximately 3 cm, then down and back to join the posterior ischial border. Together these borders form a deep notch, the greater sciatic notch, which is bounded above by the ilium and below by the ilium and ischium. The upper fibres of the sacrotuberous ligament are attached to the upper part of the posterior border. The superior rim of the notch is related to the superior gluteal vessels and nerve. The lower margin of the greater sciatic notch is covered by piriformis and is related to the sciatic nerve.

Medial border

The medial border separates the iliac fossa and the sacropelvic surface. It is indistinct near the crest, rough in its upper part, then sharp where it bounds an articular surface for the sacrum and finally rounded. The latter part is the arcuate line, which inferiorly reaches the posterior part of the iliopubic ramus, marking the union of the ilium and pubis.

• Gluteal surface

Gluteal surface is the outer surface of the ilium, which is convex in front and concave behind, like the iliac crest. It is rough and curved, convex in front, concave behind, and marked by three gluteal lines which divides into four areas. The posterior gluteal line is shortest, descending from the external lip of the crest approximately 5 cm in front of its posterior limit and ending in front of the posterior inferior iliac spine. Above, it is usually distinct, but inferiorly it is poorly defined and frequently absent. The anterior gluteal line, the longest, begins near the midpoint of the superior margin of the greater sciatic notch and ascends forwards into the outer lip of the crest, a little anterior to its tubercle. The inferior gluteal line, seldom well marked, begins posterosuperior to the anterior inferior iliac spine, curving posteroinferiorly to end near the apex of the greater sciatic notch. Between the inferior gluteal line and the acetabular margin is a rough, shallow groove. Behind the acetabulum, the lower gluteal surface is continuous with the posterior ischial surface.

The articular capsule is attached to an area adjoining the acetabular margin, most of which is covered by gluteus minimus. Posteroinferiorly, near the union of the ilium and ischium, the bone is related to piriformis.

• Iliac fossa

The iliac fossa, the internal concavity of the ilium, faces anterosuperiorly. It is limited above by the iliac crest, in front by the anterior border and behind by the medial border, separating it from the sacropelvic surface. It forms the smooth and gently concave posterolateral wall of the greater pelvis. Below it is continuous with a wide shallow groove, bounded laterally by the anterior inferior iliac spine and medially by the iliopubic ramus.

• Sacropelvic surface

The sacropelvic surface, the posteroinferior part of the medial iliac surface, is bounded posteroinferiorly by the posterior border, anterosuperiorly by the medial border, posterosuperiorly by the iliac crest and anteroinferiorly by the line of fusion of the ilium and ischium. It is divided into iliac tuberosity and auricular & pelvic surfaces. The iliac tuberosity, a large, rough area below the dorsal segment of the iliac crest, shows cranial and caudal areas separated by an oblique ridge and connected to the sacrum by the interosseous sacroiliac ligament. The sacropelvic surface gives attachment to the posterior sacroiliac ligaments and, behind the auricular surface, to the interosseous sacroiliac ligament. The iliolumbar ligament is attached to its anterior part. The auricular surface, immediately anteroinferior to the tuberosity, articulates with the lateral sacral mass. Shaped like an ear, its widest part is anterosuperior, and its ‘lobule’ posteroinferior and on the medial aspect of the posterior inferior spine. Its edges are well defined but the surface, though articular, is rough and irregular. It articulates with the sacrum and is reciprocally shaped. The anterior sacroiliac ligament is attached to its sharp anterior and inferior borders. The narrow part of the pelvic surface, between the auricular surface and the upper rim of the greater sciatic notch, often shows a rough pre-auricular sulcus (that is usually better defined in females) for the lower fibres of the anterior sacroiliac ligament. The pelvic surface is anteroinferior to the acutely curved part of the auricular surface, and contributes to the lateral wall of the lesser pelvis. Its upper part, facing down, is between the auricular surface and the upper limb of the greater sciatic notch. Its lower part faces medially and is separated from the iliac fossa by the arcuate line. Anteroinferiorly, it extends to the line of union between the ilium and ischium. Though usually obliterated, it passes from the depth of the acetabulum to approximately the middle of the inferior limb of the greater sciatic notch.

Muscle attachments

The attachment of sartorius extends down the anterior border below the anterior superior iliac spine.

The iliac crest gives attachment to the anterolateral and dorsal abdominal muscles, and to the fasciae and muscles of the lower limb.

The fascia lata and iliotibial tract are attached to the outer lip and tubercle of its ventral segment.

Tensor fasciae latae is attached anterior to the tubercle. The lower fibres of external oblique and, just behind the summit of the crest, the lowest fibres of latissimus dorsi are attached to its anterior two-thirds. A variable interval exists between the most posterior attachment of external oblique and the most anterior attachment of latissimus dorsi, and here the crest forms the base of the lumbar triangle through which herniation of abdominal contents may rarely occur.

Internal oblique is attached to the intermediate area of the crest.

Transversus abdominis is attached to the anterior two-thirds of the inner lip of the crest, and behind this to the thoracolumbar fascia and quadratus lumborum. The highest fibres of gluteus maximus are attached to the dorsal segment of the crest on its lateral slope.

Erector spinae arises from the medial slope of the dorsal segment.

The straight head of rectus femoris is attached to the upper area of the anterior inferior spine.

Some fibres of piriformis are attached in front of the posterior inferior spine on the upper border of the greater sciatic foramen.

The gluteal surface is divided by three gluteal lines into four areas. Behind the posterior line, the upper rough part gives attachment to the upper fibres of gluteus maximus and the lower, smooth region to part of the sacrotuberous ligament and iliac head of piriformis. Gluteus medius is attached between the posterior and anterior lines, below the iliac crest, and gluteus minimus is attached between the anterior and inferior lines.

The fourth area, below the inferior line, contains vascular foramina. The reflected head of rectus femoris attaches to a curved groove above the acetabulum.

Iliacus is attached to the upper two-thirds of the iliac fossa and is related to its lower one-third. The medial part of quadratus lumborum is attached to the anterior part of the sacropelvic surface, above the iliolumbar ligament.

Piriformis is sometimes partly attached lateral to the pre-auricular sulcus, and part of obturator internus is attached to the more extensive remainder of the pelvic surface.

Vascular supply Branches of the iliolumbar artery run between iliacus and the ilium; one or more enter large nutrient foramina lying posteroinferiorly in the iliac fossa. The superior gluteal, obturator and superficial circumflex iliac arteries contribute to the periosteal supply. The obturator artery may supply a nutrient branch. Vascular foramina on the ilium underlying the gluteal muscles may lead into large vascular canals in the bone. Innervation The periosteum is innervated by branches of nerves that supply muscles attached to the bone, the hip joint and the sacroiliac joint.

OSSIFICATION

Ossification is by three primary centers: one each for the ilium, ischium and pubis. The iliac centre appears above the greater sciatic notch prenatally at about the 9^th week and the pubic centre in its superior ramus between the 4^th and 5^th months. The pubis is often not recovered from prenatal remains due to its size and fragility and because it is the last of the hip bones to begin ossification (Scheuer and Black 2004). At birth the whole iliac crest, the acetabular floor and the inferior margin are cartilaginous. Gradual ossification of the three components of the acetabulum results in a triradiate cartilaginous stem extending medially to the pelvic surface as a Y-shaped epiphysial plate between the ilium, ischium and pubis, and including the anterior inferior iliac spine. Cartilage along the inferior margin also covers the ischial tuberosity, forms conjoined ischial and pubic rami and continues to the pubic symphysial surface and along the pubic crest to the pubic tubercle. The ossifying ischium and pubis fuse to form a continuous ischiopubic ramus at the 7^th or 8^th year. 

Secondary centres, other than for the acetabulum, appear at about puberty and fuse between the 15^th and 25^th years. There are usually two for the iliac crest (which fuse early), and single centres for anterior inferior iliac spine (although it may ossify from the triradiate cartilage) and symphysial surface of the pubis (the pubic tubercle and crest may have separate centres). Progression of ossification of the iliac crest in girls is an index of skeletal maturity and is useful in determining the optimal timing of surgery for spinal deformity. Between the ages of 8 and 9 years, three major centres of ossification appear in the acetabular cartilage. The largest appears in the anterior wall of the acetabulum and fuses with the pubis, the second in the iliac acetabular cartilage superiorly, fusing with the ilium, and the third in the ischial acetabular cartilage posteriorly, fusing with the ischium. At puberty, these epiphyses expand towards the periphery of the acetabulum and contribute to its depth. Fusion between the three bones within the acetabulum occurs between the sixteenth and eighteenth years. Delaere et al have suggested that ossification of the ilium is similar to that of a long bone, possessing three cartilaginous epiphyses and one cartilaginous process, although it tends to undergo osteoclastic resorption comparable with that of cranial bones. During development, the acetabulum increases in breadth at a faster rate than it does in depth. Avulsion fractures of pelvic apophyses may occur from excessive pull on tendons, usually in athletic adolescents. The most frequent examples of such injuries are those to the ischial tuberosity (hamstrings) and anterior inferior iliac spine (rectus femoris).

Complied & written by Dr. Palak Shah.

 

HAND PHALANGES

The phalanges are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones.

Each proximal phalanx consists of three parts:

• The base, which represents the expanded proximal part. It has a concave, oval-shaped articular facet that articulates with the metacarpal head to form the metacarpophalangeal (MCP) joint. The base also contains nonarticular tubercles for the attachment of various soft tissue structures.
• The body, which continues distally from the base. It tapers distally and has two surfaces: dorsal and palmar. The dorsal surface is round and smooth, appearing convex in the transverse plane. The palmar surface is flat and rough, especially on the sides where the flexor fibrous sheaths of digits attach. The surface appears flat in the transverse plane but concave in the sagittal plane.
• The head, which represents the expanded and rounded distal part. It has a pulley-shaped articular surface that articulates with the base of the middle phalanx to form the proximal interphalangeal (PIP) joint. The heads consist of smooth grooves, especially on the palmar aspects. These grooves represent the attachment points of the collateral interphalangeal ligaments of hand.

Various ligaments attach to the proximal phalanges. The most complex one is the digital fascial complex which attaches the surrounding subcutaneous tissue and neurovasculature to the bony phalanges. The collateral and palmar metacarpophalangeal ligaments attach to the bases of the proximal phalanges. They provide strength to the metacarpophalangeal joints. The collateral interphalangeal ligaments of hand attach to the heads, supporting the PIP joints. The proximal phalanges are also covered by the extensor expansion of hand on the dorsal aspect.

The proximal phalanges are very mobile at the MCP joints. They are mainly capable of flexion, extension, adduction and abduction. Circumduction and rotation are also possible, especially at the MCP joint of the thumb. These movements are enabled by the action of several muscles:

• Posterior (extensor) forearm muscles, such as extensor digitorum, extensor pollicis brevis, extensor digiti minimi and extensor indicis.
• Metacarpal muscles, such as the lumbricals, palmar interossei and dorsal interossei.
• Thenar muscles, for example flexor pollicis brevis and adductor pollicis.
• Hypothenar muscles like abductor digiti minimi and flexor digiti minimi.

These muscles carry out their functions via their direct attachments to the bases of the proximal phalanges. In addition, many extensors carry out the movements via the extensor expansion of hand which covers the phalanges

Middle phalanges

There are four middle (intermediate) phalanges in each hand because the thumb is missing one. They have a similar structure to the proximal ones, consisting of a base, body and head. The base of each middle phalanx has two concave-shaped articular facets and matches the head of the corresponding proximal phalanx. Their apposition forms the PIP joint. The heads of the middle phalanges have a pulley-like appearance. They articulate with the bases of the distal phalanges to form the distal interphalangeal (DIP) joints of hand.

The middle phalanges are reinforced by the same ligaments supporting the proximal ones, digital fascial complex, collateral interphalangeal ligaments and extensor expansion of hand. The collateral interphalangeal ligaments attach to the base and heads of the middle phalanges to reinforce the PIP and DIP joints.

The middle phalanges are less mobile compared to the proximal phalanges. They are only capable of flexion and extension at the PIP joints. Only the flexor digitorum superficialis muscle attaches directly to the sides of the middle phalanges, flexing them at the PIP joints. The remaining contributions are provided by the action of the previously mentioned muscles; the forearm extensors, metacarpal, thenar and hypothenar muscle groups. Flexion and extension are transferred to the middle phalanges from the direct action of these muscles on the proximal phalanges or via the extensor expansion of hand.

Distal phalanges

Each hand has five distal phalanges, which look shorter and slightly thicker compared to the previous two sets. Each distal phalanx has a base, body and head. The base has a double articular facet which matches the shape of the head of the middle phalanx. The distal phalanges have a smooth and round dorsal surface. In contrast, their palmar surface is wrinkled and irregular. The nonarticular heads contain an irregular, curved shaped distal tuberosity. It serves as an anchor point for the pulps of the digits.

The distal phalanges are stabilized by the digital fascial complex, collateral interphalangeal ligaments and extensor expansion of hand. The collateral interphalangeal ligaments attach to the base of the distal phalanges to reinforce the DIP joints.

The distal phalanges are capable of flexion and extension at the DIP joints. Two forearm extensors and one flexor muscle insert directly into the bases of the distal phalanges, permitting these actions. These include flexor digitorum profundus, flexor pollicis longus and extensor pollicis longus. The previously mentioned muscle groups acting on the proximal and middle phalanges also act indirectly on the distal ones via the extensor expansion of hand.

BLOOD SUPPLY

The hand phalanges are richly supplied with blood, lymphatics, and nerves, and their development involves a precise pattern of ossification. The blood supply to the phalanges comes primarily from the digital arteries, which are branches of the superficial and deep palmar arches derived from the radial and ulnar arteries. These arteries run alongside the phalanges, especially near the lateral aspects, where they give off perforating branches that penetrate the bone through nutrient foramina. Venous drainage mirrors the arterial supply, with the digital veins draining into the superficial and deep venous systems of the hand. The lymphatic drainage follows the venous pathways, with lymphatic vessels accompanying the digital veins. These vessels drain into the epitrochlear and axillary lymph nodes, playing a crucial role in immune surveillance and fluid balance in the hand.

NERVOUS SUPPLY

The nervous supply of the phalanges is derived from the median, ulnar, and radial nerves, which innervate the hand based on their anatomical distribution. The median nerve supplies the palmar side of the first three and a half fingers and their corresponding phalanges, while the ulnar nerve supplies the remaining fingers. The radial nerve provides sensation to the dorsal aspect of the phalanges, primarily for the proximal portions of the first three fingers. These nerves are responsible for transmitting sensory information, including pain, touch, and temperature, and they also play a critical role in motor function by innervating the muscles controlling finger movement.

OSSIFICATION

Ossification of the hand phalanges follows a well-defined sequence. Each phalanx typically ossifies from one primary ossification center, which appears during fetal development, generally between the 8th and 12th weeks of gestation. A secondary ossification center forms at the base of the phalanx during early childhood, usually between 2 and 4 years of age, depending on the specific phalanx and its position in the hand. The fusion of the primary and secondary ossification centers, marking skeletal maturity, occurs by 15–18 years of age. The ossification sequence begins with the proximal phalanges, followed by the middle and distal phalanges. This progression is vital for assessing growth and development in pediatric radiology and clinical evaluations.

RADIUS

The radius or radial bone is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna. The ulna is usually slightly longer than the radius, but the radius is thicker. Therefore, the radius is considered to be the larger of the two. It is a long bone, prism-shaped and slightly curved longitudinally.

The word radius is Latin for "ray". In the context of the radius bone, a ray can be thought of rotating around an axis line extending diagonally from center of capitulum to the center of distal ulna. While the ulna is the major contributor to the elbow joint, the radius primarily contributes to the wrist joint.

The radius is named so because the radius (bone) acts like the radius (of a circle). It rotates around the ulna and the far end (where it joins to the bones of the hand), known as the styloid process of the radius, is the distance from the ulna (center of the circle) to the edge of the radius (the circle). The ulna acts as the center point to the circle because when the arm is rotated the ulna does not move.

The radius is part of two joints: the elbow and the wrist. At the elbow, it joins with the capitulum of the humerus, and in a separate region, with the ulna at the radial notch. At the wrist, the radius forms a joint with the ulna bone.

The radius has an upper end, a shaft and a lower end.

GENERAL FEATURES 

Upper End

Head is disc shaped and is covered with a Hyaline cartilage. It has a superior concave surface which articulates with the capitulum of the humerus at the elbow joint. The circumference of the head is smooth; it is broad medially where it articulates with the radial notch of the ulna, narrow in the rest of its extent and then embraced by the annular ligament thus forming a superior radioulnar joint which provides supination and pronation movement.

Neck is enclosed by the narrow lower margin of the annular ligament. The head and neck are free from capsular attachment and can rotate freely with the socket

Tuberosity lies just below the medial part of the neck. It has a rough posterior and a smooth anterior part.

Shaft

The shaft of radius is prismoid in form, narrower above than below, and slightly curved, so as to be convex lateralward. It presents three borders and three surfaces.

Borders 

1. Anterior or Volar border extends from the lower part of the tuberosity above to the anterior part of the base of the styloid process below and separates the volar from the lateral surface. It is oblique in the upper half of the shaft and vertical in the lower half. The oblique part is called the anterior oblique line and lower part is crest like.

2. Posterior border begins above at the back of the neck and ends below at the posterior part of the base of the styloid process; it separates the posterior from the lateral surface. is indistinct above and below, but well-marked in the middle third of the bone.

3. Medial or Interosseous border is the sharpest of the three borders which extends from the radial tuberosity above to the posterior margin of the ulnar notch below. The interosseous membrane is attached to its lower 3/4th. In its lower part it forms the posterior margin of an elongated triangular area.

Surfaces 

1. Anterior surface lies between the anterior and interosseous borders. A nutrient foramen opens in its upper part which is directed upwards. Nutrient artery is a branch of the anterior interosseous artery.

2. Posterior surface lies between posterior and interosseous borders.

3. Lateral surface lies between the anterior and posterior borders.

Lower End

The lower end is the widest part of the bone. It has 5 surfaces:

1.The anterior surface is in the form of a thick prominent ridge. The radial artery is palpated against this surface.

2.The posterior surface presents four grooves for the extensor tendons. The dorsal tubercle (of Lister) lies lateral to an oblique groove.

3.The medial surface is occupied by the ulnar notch for the head of the ulna.

4.The lateral surface is prolonged downwards to form the styloid process.

5.The inferior surface bears a triangular area for the scaphoid bone, and a medial quadrangular area for the lunate bone. This surface takes part in forming the wrist joint.

ATTCHMENTS TO MUSCLES & LIGAMENTS AND JOINTS

The lateral ligaments of the elbow are collectively named the lateral collateral ligament complex, which is composed of four ligaments that are difficult to separate. First, the lateral collateral ligament (a.k.a. "radial collateral ligament") attaches from the lateral epicondyle of the humerus to the annular ligament. The lateral collateral ligament joins the radius to the humerus and protects the antebrachium against varus stress. The annular ligament attaches from anterior to the posterior radial notch of the ulna encircling the radial head allowing pronation and supination. The lateral ulnar collateral ligament attaches from the lateral epicondyle of the humerus to the ulnar supinator crest. The lateral ulnar collateral ligament helps protect the antebrachium against valgus stress. Finally, the accessory lateral collateral ligament attaches from the supinator crest to the inferior margin of the annular ligament, providing further stability to the joint.

Between the medial border of the radius and the lateral border of the ulna resides the interosseous membrane which divides the anterior and posterior compartments of the antebrachium. The interosseus membrane serves as an attachment site for muscle while allowing for force distribution across the forearm. When the forearm supinates, the interosseous membrane fibers become taught, stabilizing distal and proximal joints. Like the lateral collateral ligament complex, the composition of the interosseous membrane is of multiple ligaments that are difficult to separate. The five ligaments of the interosseous membrane are the proximal oblique cord, accessory band, central band, dorsal oblique accessory cord, and the distal oblique bundle.

The distal radioulnar articulation is composed of the palmar radioulnar ligament (volar radioulnar ligament) that attaches the anterior radius to the anterior ulna, the dorsal radioulnar ligament (posterior radioulnar ligament) which attaches the posterior radius to the posterior ulna, and the articular disc which lies between the distal ulna and radius. These structures allow for load distribution and allow the second pivot during pronation and supination of the forearm.

The distal radius is attached to the lunate bone via two ligaments, the long and short radiolunate ligaments. These ligaments prevent hyperextension of the lunate bone during wrist extension. The distal radius is attached to the scaphoid via the radial collateral ligament of the wrist, and the radioscaphocapitate ligament attaches the volar aspect of the radius to the scaphoid and capitate carpal bones. Lastly, the dorsal radiotriquetral ligament is a wide ligament, which connects the dorsal radius to the dorsal scaphoid, lunate, and triquetrum.

The muscles of the forearm divide into two compartments - posterior and anterior. The posterior compartment is composed primarily of muscles that allow for wrist extension, finger extension, and forearm supination. The anterior compartment contains mostly the muscles involved in wrist flexion, finger flexion, and forearm pronation. 

The anterior compartment subdivides into the superficial, intermediate, and deep layers. The muscles of the superficial layer are the flexor carpi ulnaris, palmaris longus, flexor carpi radialis, and pronator teres. The intermediate layer contains a single muscle, the flexor digitorum superficialis. The deep anterior layer is composed of the flexor digitorum profundus, the flexor pollicis longus, and the pronator quadratus.

The posterior compartment of the forearm subdivides into superficial and deep layers. The muscles of the superficial layer are the brachioradialis, extensor carpi radialis brevis, extensor carpi radialis longus, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and anconeus muscles. The muscles of the posterior layer are supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis muscles.

BLOOD SUPPLY, LYMPHATICS AND NERVES

The blood supply to the proximal radius is provided primarily by the radial and radial recurrent arteries. The radial recurrent artery travels proximally to anastomose with the radial collateral artery. Blood supply to the middle and distal radius is through the radial, posterior interosseous, and anterior interosseous arteries.

Lymph from the radius drains via the deep lymphatic vessels which follow the deep veins such as the brachial, ulnar, and radial veins.  These vessels drain into the humeral axillary lymph nodes.

The nerves of the brachial plexus provide motor and sensory innervation to the antebrachium. 

The radial nerve provides sensory innervation for the posterior forearm and motor innervation to the brachioradialis, extensor carpi radialis brevis, extensor carpi radialis longus, supinator, extensor carpi ulnaris, abductor pollicis longus, abductor pollicis brevis, extensor pollicis longus, extensor pollicis brevis, extensor indicis, extensor digitorum, and extensor digiti minimi muscles. 

The medial and lateral antebrachial cutaneous nerves provide sensory innervation to the anteromedial and anterolateral forearm respectfully.

The musculocutaneous nerve is the source of motor innervation to the biceps brachii.

The median nerve provides motor innervation to the pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, flexor pollicis longus, pronator quadratus, and the lateral half of the flexor digitorum profundus muscles.

The ulnar nerve supplies motor innervation to the flexor carpi ulnaris and flexor digitorum profundus muscles.

EMBRYOLOGY AND OSSIFICATION

The development of the radius occurs through endochondral ossification and begins with lateral plate mesoderm - the origin of all long bones. Endochondral ossification is the replacement of hyaline cartilage with bone. Mesoderm-derived mesenchymal cells become chondrocytes, which proliferate rapidly and form a bone template. 

Chondrocytes near the center of the model begin laying down collagen and fibronectin to the matrix which allows for calcification. Around week six of gestation, the calcification prevents nutrients from reaching the chondrocytes. The chondrocytes eventually undergo apoptosis. Blood vessels proliferate through the spaces vacated by the chondrocytes and eventually form the medullary cavity. Around the 12th week of gestation, osteoblasts create a thick area of compact bone in the diaphysis called the periosteal collar which becomes the site of primary ossification. Epiphyseal plates (growth plates) at either end of the bone contain proliferating chondrocytes, that continue to elongate the bone, until puberty. During puberty, sex hormones cause secondary ossification centers to form, and the epiphyseal plates ossify in a congruent fashion as the diaphysis.

Complied & Written by Dr. Palak Shah

CARPAL BONES

 The Latin word "carpus" is derived from Greek καρπὁς meaning "wrist". The root "carp-" translates to "pluck", an action performed by the wrist.  In human anatomy, the main role of the wrist is to facilitate effective positioning of the hand and powerful use of the extensors and flexors of the forearm, and the mobility of individual carpal bones increase the freedom of movements at the wrist. 

There are 8 carpal bones, organized into two longitudinal rows, the proximal row contains Scaphoid, 

Lunate, Triquetrum and Pisiform and the distal row has Trapezium, Trapezoid, Capitate and 

Hamate.

SCAPHOID

It is a boat shaped bone and has a tubercle which is laterally, forwards and downwards.

The tubercle of the scaphoid gives attachment to flexor retinaculum and a few fibres of 

abductor pollicis brevis.

Scaphoid articulates with Radius, Lunate, Capitate, Trapezium and Trapezoid.

LUNATE

It is half-moon shaped or crescentic bone.

As it has a semi lunar surface, it articulates with the scaphoid on its lateral side. A quadrilateral 

surface for the triquetral on its medial side.

TRIQUETRAL 

It is a pyramid shaped bone.

The oval facet for the pisiform lies on the distal part of the palmar surface.

The medial and dorsal surfaces are continuous and nonarticular.

It articulates with pisiform, lunate, hamate and articular disc of the inferior radioulnar joint.

PISIFORM

It is a pea shaped bone.

The oval facet for the triquetral lies on the proximal part of the dorsal surface.

The lateral surface is grooved by the ulnar nerve.

It only articulates with Triquetral.

TRAPEZIUM

It is quadrangular in shape, has a crest and a groove anteriorly.

The palmar surface has a vertical groove for the tendon of the flexor carpi radialis.

The groove is limited literally by the crest of the trapezium.

The distal surface bears convexo-concave articular surface for the base of the 1st metacarpal bone.

It articulates with scaphoid, trapezium, capitate and 1st and 2nd metacarpal.

TRAPEZOID

It looks like a shoe of a baby.

The distal articular surface is bigger than the proximal.

The palmar nonarticular surface is prolonged laterally.

It articulates with scaphoid, trapezium, 2nd metacarpal and capitate.

CAPITATE

It is the largest bone in the carpal bones which has a rounded head.

The dorsomedial angle is the distal most projection from the body of the bone which bears a small facet for the 4th metacarpal bone.

It articulates with scaphoid, lunate, hamate, trapezoid and 2nd, 3rd & 4th metacarpals.

HAMATE

It is a wedge shaped with a hook near its base.

The hook projects from the distal parts of the palmar surface and is directed laterally.

MUSCLES AND LIGAMENTS

The intricate movements of the hand are facilitated by a delicate balance of muscular forces and a robust ligamentous network within the wrist. Two primary muscle groups contribute: extrinsic muscles, originating in the forearm, stabilize the wrist by maintaining hand position on the radius during coordinated muscle contractions. Intrinsic muscles, originating within the hand, fine-tune movements by balancing flexor and extensor forces.

The flexor carpi ulnaris, an extrinsic muscle, significantly influences wrist movement by inserting onto the hamate, pisiform, and the base of the fifth metacarpal. Intrinsic muscles demonstrate diverse origins: thenar muscles arise from the scaphoid and trapezium, the adductor pollicis originates from the capitate and second/third metacarpals, and hypothenar muscles originate from the pisiform and hamate.

A complex system of ligaments provides stability. Radiocarpal ligaments connect the radius to various carpal bones (scaphoid, lunate), while ulnocarpal ligaments connect the ulna to the lunate and capitate. Intercarpal ligaments bind the carpal bones together (e.g., lunotriquetral, scapholunate), forming a strong, interconnected structure. These ligaments, along with carpometacarpal and intermetacarpal ligaments, ensure stability during a wide range of hand movements.

BLOOD SUPPLY, LYMPHATICS AND NERVOUS SUPPLY

The radial artery, ulnar artery, and their anastomoses provide the blood supply of the wrist. The radial artery predominantly supplies the thumb and the lateral side of the index finger while the ulnar artery supplies the rest of the digits and the medial side of the index finger. In particular, vascular supply takes place via the anastomotic network consisting of three dorsal and three palmar arches, which arise from both the radial and ulnar arteries, that overlie the carpal bones. The scaphoid, capitate, and a minority of lunates (20%) all have one intraosseous vessel supply. Of note, the scaphoid has a single blood supply from the radial artery that enters from the distal portion of the bone to supply the proximal portion, thus making its proximal pole most vulnerable to avascular necrosis. The trapezoid and hamate both have two areas of blood supply without intraosseous anastomoses. The trapezium, triquetrum, pisiform, and most lunates (80%) have two areas of blood supply and consistent intraosseous anastomoses. Therefore, the rest of the carpal bones, excluding the scaphoid, capitate, and the minority of lunates, have a lower risk of developing avascular necrosis following a fracture.

Innervation of the wrist joint comes from the:

• anterior interosseous branch of the median nerve
• posterior interosseous branch of the radial nerve
• the dorsal and the deep branches of the ulnar nerve

The lateral antebrachial cutaneous nerve, the posterior interosseous nerve, the dorsal branch and the perforating branches of the ulnar nerve, and the superficial branch of the radial nerve innervate the wrist joint from the dorsum. The palmar cutaneous branch of the median nerve, the anterior interosseous nerve, and the main trunk and deep branch of the ulnar nerve innervate the wrist joint from the palmar side.

EMBRYOLOGY

Upper limb development initiates with the activation of a group of mesenchymal cells in the lateral mesoderm towards the end of the fourth week, with the limb buds becoming visible around day 26 or day 27. Each limb bud comprises a mass of mesenchyme covered by ectoderm. This mesenchyme remains undifferentiated until it is ready to develop into bone, cartilage, and blood vessels later in development. Meanwhile, at the apex of each limb bud, the ectoderm thickens to form the apical ectodermal ridge, which stimulates the growth and development of the upper limb bud in the proximal-distal axis. Other signaling centers and primary morphogens such as the zone of polarizing activity, derived from an aggregate of mesenchymal cells in the limb bud, and the Want pathway, expressed from the dorsal epidermis of the limb bud, also contribute to the development of the upper limb buds by regulating growth along the anteroposterior axis and the dorsoventral axis, respectively.

At the end of the sixth week of development, digital rays form in the hand plate. By the seventh week, the carpal chondrification process begins. The capitate and the hamate carpal bones are the first chondrogenic centers to appear as immature cartilage early in the eighth week while the pisiform is the last to appear later in the eighth week. The hamulus, otherwise known as the “hook of the hamate,” also appears as an immature cartilaginous tissue towards the end of the eighth week and does not complete its development until the thirteenth week. Last, in the fourteenth week, a vascular bud penetrates the lunate cartilage mold, an early sign of the osteogenic process that will complete during the first year of life.

Complied & Written by Dr. Palak Shah

ULNA

The ulna is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. It runs parallel to the radius, the other long bone in the forearm. The ulna is usually slightly longer than the radius, but the radius is thicker.

The ulna has a bony process, the olecranon process, a hook-like structure that fits into the olecranon fossa of the humerus. This prevents hyperextension and forms a hinge joint with the trochlea of the humerus. There is also a radial notch for the head of the radius, and the ulnar tuberosity to which muscles attach.

The long, narrow medullary cavity of the ulna is enclosed in a strong wall of cortical tissue which is thickest along the interosseous border and dorsal surface. At the extremities the compact layer thins. The compact layer is continued onto the back of the olecranon as a plate of close spongy bone with lamella parallel. From the inner surface of this plate and the compact layer below its trabecula arch forward toward the olecranon and coronoid and cross other trabecula, passing backward over the medullary cavity from the upper part of the shaft below the coronoid. Below the coronoid process there is a small area of compact bone from which trabecula curve upward to end obliquely to the surface of the semilunar notch which is coated with a thin layer of compact bone. The trabecula at the lower end has a more longitudinal direction.

The ulna has an upper end, shaft and a lower end.

GENERAL FEATURES

Upper End

It presents the olecranon and coronoid processes and the trochlear and radial notches.

1. Olecranon process projects upwards from the shaft. It has 5 surfaces. Anterior surface is articular which forms the upper part of the trochlear notch. Posterior surface forms a triangular subcutaneous area which is separated from the skin by bursa. Inferiorly it is continuous with the posterior border of the shaft of the ulna and upper part forms the point of the elbow. Medial surface is continuous inferiorly with the medial surface of the shaft. Lateral surface is smooth and Superior surface in its posterior part shows a roughened area.

2. Coronoid process projects forwards from the shaft just below the olecranon and has 4 surfaces. Superior surface forms the lower part of the trochlear notch. Anterior surface is triangular and rough & its lower corner forms the ulnar tuberosity. Lateral surface upper part is marked by radial notch for the head the radius. Annular ligament is attached to the anterior and posterior margins of the notch. Lower part forms a depressed area to accommodate the radial tuberosity. It is limited behind by a ridge called supinator crest. Medial surface is continuous with the medial surface of the shaft.

3. Trochlear notch forms an articular surface which articulates with trochlea of the humerus to form the elbow joint.

4. Radial notch articulates with the head of the radius to form the superior radioulnar joint.

Shaft

The shaft of the Ulna at its upper part is prismatic in form and curved so as to be convex behind and lateralward; its central part is straight; its lower part is rounded, smooth, and bent a little lateralward. It tapers gradually from above downward and has three borders and three surfaces.

Borders

1.The interosseous or lateral border is sharpest in its middle two-fourths. Inferiorly, it can be traced to the lateral side of the head. Superiorly, it is continuous with the supinator crest.

2.The anterior border is thick and rounded. It begins above on the medial side of the ulnar tuberosity, passes backwards in its lower one-third, and terminates at the medial side of the styloid process.

3.The posterior border is subcutaneous. It begins, above, at the apex of the triangular subcutaneous area at the back of the olecranon and terminates at the base of the styloid process.

Surfaces

1.The anterior surface lies between the anterior and interosseous borders. A nutrient foramen is seen on the upper part of this surface. It is directed upwards. The nutrient artery is derived from the anterior interosseous artery.

2.The medial surface lies between the anterior and posterior borders.

3.The posterior surface lies between the posterior and interosseous borders. It is subdivided into three areas by two lines. An oblique line divides it into upper and lower parts. The lower part is further divided by a vertical line into a medial and a lateral area.

Lower End 

The lower end of the ulna presents an articular surface, part of which, of an oval or semilunar form, is directed downward, and articulates with the upper surface of the triangular articular disc which separates it from the wrist-joint; the remaining portion, directed lateralward, is narrow, convex, and received into the ulnar notch of the radius.

Near the wrist, the ulnar, with two eminences; the lateral and larger is a rounded, articular eminence, termed the head of the ulna; the medial, narrower and more projecting, is a non-articular eminence, the styloid process.

The head presents an articular surface, part of which, of an oval or semilunar form, is directed downward, and articulates with the upper surface of the triangular articular disk which separates it from the wrist-joint; the remaining portion, directed lateralward, is narrow, convex, and received into the ulnar notch of the radius. 

The styloid process projects from the medial and back part of the bone; it descends a little lower than the head, and its rounded end affords attachment to the ulnar collateral ligament of the wrist joint.

The head is separated from the styloid process by a depression for the attachment of the apex of the triangular articular disk, and behind, by a shallow groove for the tendon of the extensor carpi ulnaris.

PARTICULAR FEATURES:

MUSCLES

The ulna serves as the attachment site for numerous muscles with a myriad of actions. The following are organized in terms of the direction and where on the ulna is the attachment of the muscle’s fibers.

The following muscles insert into the ulna:

Triceps brachii – posterior section of the superior surface of the olecranon Anconeus – olecranon Brachialis – the volar surface of the coronoid process

The following muscles originate from the ulna:

• Pronator teres – the medial surface of the coronoid process
• Flexor carpi ulnaris – olecranon process
• Flexor digitorum superficialis – coronoid process
• Flexor digitorum profundus – anteromedial surface
• Pronator quadratus – distal anterior shaft
• Extensor carpi ulnaris – posterior border
• Supinator – proximal ulna
• Abductor pollicis longus – posterior surface
• Extensor pollicis longus – dorsal shaft
• Extensor indicis – posterior distal shaft
  

Blood vessels and Lymphatics

The main blood supply of the ulna originates from the ulnar artery or the ulnar recurrent artery. The ulnar artery then branches into a common interosseous artery that then further subdivides into the posterior and anterior interosseous vessels. These vessels are vital for the nutrients of the ulnar diaphysis. The ulnar metaphysis is supplied by the end branches of the anterior interosseous artery, while the head of the ulna receives its supply from small divisions off of the ulnar artery proper.

The lymphatics of the ulna drain either to the supratrochlear lymph node or directly travel to the adjacent cluster of axillary lymph nodes.

Nerves

Three main nerves run in the forearm in the proximity of the ulna: the median, ulnar, and radial nerves. The median nerve runs anterolaterally and innervates the muscles responsible for flexion of the wrist and the fingers (digits 1 to 3 and radial aspect of the fourth). The ulnar nerve is positioned more anteromedially, running in between the flexor digitorum superficialis and flexor digitorum profundus. This nerve is also involved in flexion of the wrist and fingers (the ulnar aspect of 4th and all of the 5th) but also is responsible for spanning the fingers. A major branch of the median nerve is the anterior interosseous nerve. Impingement or entrapment of the anterior interosseous nerve results in the characteristic clinical finding of weakness related to the "pincer" movement. The radial nerve extends posteriorly to the ulna and the radius and is the sole nerve involved in the muscles responsible for extension of the forearm, wrist, and fingers.  The posterior interosseous nerve is a branch of the radial nerve with both motor and sensory functions. Posterior interosseous nerve impingement or entrapment occurs in the region of the radial tunnel resulting in weakness related to the extension of the digits.

EMBRYOLOGY AND OSSIFICATION

The ulna develops from the induction of the lateral plate mesoderm. From this, it differentiates into the somatic mesoderm, which, in turn, gives rise to all of the bones, ligaments, connective tissue, and blood vessels of the extremities. Endochondral ossification allows for the induction of bone from previously laid hyaline cartilage. The ulna itself is ossified in three sections. First, it commences at the body of the ulna. Around week eight of gestation, the chondrocytes in the middle of the ulna lay down collagen and fibronectin to appropriately calcify the bone template. At the time of birth, the ends of the bone are still cartilaginous and not ossified. Around age four, an ossification center arises in the middle of the ulnar head and then encompasses into the styloid process. At age ten, a subsequent center appears in the olecranon, which is then met by the growth of the epiphysis at approximately age sixteen. The ossification centers about the elbow occur in a predictable order and understanding/knowledge of these stages of development is essential for identifying traumatic injuries about the elbow